The present invention relates to a transmission (T)/reception (R) source antenna, called hereafter a T/R source, that can be placed at the focal point of an antenna system and more particularly at the focal point of a Cassegrain-type double-reflector antenna. One possible application for this T/R source is in satellite communication systems using the C-, Ku- or Ka-bands.
In French Patent Application No. 00/07424 filed on Jun. 9, 2000 in the name of Thomson Multimedia, entitled “Perfectionnement aux antennes-source d'émission/réception d'ondes électromagnétiques”, [Improvement to electromagnetic wave transmission/reception source antennas], a hybrid T/R source has been proposed which consists of an array of helices that is excited by an printed feed circuit, surrounding a longitudinal-radiation antenna such as a helix or a “polyrod”.
To minimize the interactions between the transmission and reception sources, it is advantageous to use the array of helices for reception and the longitudinal-radiation source for transmission. However, in reception, the losses of the impressed feed circuit have a double effect on the link budget. This is because the G/T ratio of merit of the antenna is reduced because, on the one hand, of the reduction in the gain G of the antenna and, on the other hand, of the increase in the noise temperature T of the system owing to the dissipative losses of the feed circuit. From this standpoint, the solution proposed in Patent Application 00/07424 makes it possible, using an array of helices, preferably with an array of patches, to improve the G/T ratio of the antenna.
Moreover, in French Patent Application 00/07424, the substrate on which the printed feed circuit of the helices is etched, and which includes the receiving circuits of the antenna, is placed perpendicular to the radiation axis of the helices. Thus, in a Cassegrain structure, to avoid blocking by the LNB (Low Noise Block), it is necessary to place the focus of the double reflector system at the apex of the main reflector. This constraint on the geometry of the Cassegrain system requires the use of an overly directional source, which has the effect of increasing the level of the side lobes of the antenna system.
This is because, as illustrated in FIG. 1 which shows schematically a Cassegrain structure comprising a main reflector 1, a source 2 and a secondary reflector 3 facing the source 2, the side lobes principally arise from:                i) the diffraction by the secondary reflector 3. The diffracted energy has an absolute level in dB equal to (G-Edge). G is the gain of the primary source defined essentially by its directivity. For optimum operation of the double-reflector antenna system, Edge is around 20 dB. The level of the side lobes resulting from this diffraction is around the value of (G-Edge);        ii) the side lobes I radiated by the same source 2 and not intercepting the secondary reflector 3. If the primary source 1 has a side lobe level in dB equal to SLL, then the absolute level of the side lobes of the antenna system resulting from the side lobes of the primary source is equal to (G-SLL).        
One solution for reducing the lobes of a Cassegrain system is to reduce G. However, as illustrated in FIG. 2, to reduce G and keep an optimum Edge value (of around 20 dB), the focal point 2′ of the antenna system must be located between the main reflector 1 and the secondary reflector 3.